A mass spectrometer measures the masses within a molecular sample to analyze the composition of the sample. A residual gas analyzer (RGA) is a relatively small mass spectrometer that measures the composition of a gas by ionizing components of the gas to create a charge, and determining the mass-to-charge ratios of those components. RGAs are commonly used to check for gas composition and contamination, and may operate in an evacuated environment at lower pressure than the source of the gas being analyzed. The main components of a residual gas analyzer are an ion source, mass analyzer (mass filter), detector and associated electronics. The ion source ionizes molecules of the gas, the mass analyzer selects the ions by their mass-to-charge ratio, and the detector determines the amounts of the selected ions.
RGA ion sources are generally one of two types: open or closed. An open ion source is usually mounted in a vacuum chamber with its components exposed to sample gas from a process environment, directly. The sample gas molecules in the vacuum chamber can move through the ion source from many directions—there is no pressure difference within the ion source and around it. When the pressure of the gases is too high for the RGA to operate properly, a pressure-reducing gas-sampling vacuum system is used to bring a sample of the gas to be analyzed down to an acceptable pressure. In such applications, an open ion source suffers from drawbacks, such as interference from the gases in the residual vacuum of the sampling system (e.g., hydrogen, water, carbon monoxide, oils).
A closed ion source is preferred, generally, when using an RGA to analyze gas with a pressure-reducing gas-sampling system. A closed ion source provides an ionization chamber operated at, or below the pressure of the sample gas, but higher than can be tolerated by the whole RGA. This chamber has restricted gas exit conductance with only small openings for entrance and exit of gases, electrons, and ions. Electrons are directed into the chamber to form ions of the sample gas at the relatively high pressure in the chamber. The sample gas is at higher pressure than could be tolerated with an open ion source, so the signal from the sample gas is correspondingly higher than the signal from the residual vacuum of the pressure reducing system, providing a higher fidelity analysis of the sample gas. Because critical electrode surfaces of the closed ion source are exposed to the sample gas at a higher pressure than an open ion source, the closed ion source is susceptible to degradation much faster because the sample gas can contaminate those surfaces. Additionally, the electron source is typically located close to the hole where electrons are introduced into the ionization chamber, and is thus exposed to the sample gas at a pressure much higher than the average pressure of the mass spectrometer. Thus, closed ion sources have higher analytical fidelity but are susceptible to higher degradation rates, while open ion sources have lower degradation rates but provide lower analytical fidelity.
Prior approaches to this degradation problem used in other (non RGA) systems include cross beam ionizers and dynamically adjusted ion sources with extra control surfaces. However, extra control surfaces increase cost and complexity, often require frequent adjustment procedures, and have limited effectiveness with extreme contamination. Cross beam ion sources have low sensitivity for the amount of gas consumed as they analyze a collimated gas stream from a small portion of the sampled gas using multistage pumping systems to strip off a majority of the sampled gas. This results in either a small sample gas signal, or the need for large, expensive pumping systems that consume high flows of sample gas.